The fiber structure of the heart plays a critical role in shaping electrical propagation. Conduction is influenced by tissue geometric factors such as expansion and contraction, and is anisotropic, with current spread being most rapid in the direction of the fiber long axis. Spatial rate of change of fiber orientation also influences conduction properties. Remodeling of ventricular geometry and fiber organization, including development of interstitial fibrosis, is a prominent feature of several cardiac pathologies, and these alterations may figure importantly in arrhythmogenesis. A detailed knowledge of ventricular fiber structure, how it may be remodeled in cardiac pathology, and the effects of this remodeling on ventricular conduction is therefore of fundamental importance to the understanding of cardiac electro-mechanics in health and disease. We will investigate how anatomical remodeling of ventricular fiber structure influences ventricular conduction, using the canine tachycardia pacing-induced heart failure preparation as a model system. Several aims must be accomplished to do this. First, we will develop MR imaging methods for the rapid reconstruction of ventricular fiber structure. Second, we will use these methods to measure fiber structure in populations of normal and failing hearts. Third, we will develop mathematical methods for identifying statistically significant changes in fiber structure between normal and failing hearts. Fourth, we will measure electrical activation patterns in each heart that is anatomically reconstructed using MR imaging methods. Fifth, we will relate measured changes in fiber structure to measured changes of electrical propagation in each heart using both experimental approaches as well as computational models.